We are studying the role of ascorbic acid and neoplastic transformation in regulating collagen metabolism in animal cells. Deficiency of ascorbate in guinea pigs leads to decreased proline hydroxylation and defective collagen synthesis. In contrast, stationary cultures of L-929 cells do not require ascorbate for proline hydroxylation and contain a substance in the microsomal membrane which can effectively hydroxylate proline during procollagen synthesis in the rough endoplasmic reticulum.